1. Field of the Invention
The present disclosure relates to a toner set, developer set, image forming apparatus, and image forming method.
2. Description of the Related Art
A copy machine, a printer, a fax machine, a combined machine thereof, and other image forming apparatus using an electrophotographic system have an image bearing member, a charging device for charging a surface of the image bearing member uniformly, an exposure device for forming an electrostatic latent image on the image bearing member, a developing device for developing the electrostatic latent image on the image bearing member to a toner image, and a transfer device for transferring the toner image on the image bearing member onto a recording medium. This image forming apparatus uses each of these devices to transfer the toner image onto a recording medium as described above, to form an image on the recording medium.
Such an image forming apparatus has not only a function for printing in black and white, but also a color printing function for printing color images. Specifically, there is used an image forming apparatus with a single photosensitive drum, such as a single-drum type color copy machine or multi-functional peripheral (MFP). However, when printing a color image on a single sheet, this kind of single-drum type image forming apparatus needs to form the image on the photosensitive drum serving as an image bearing member, each time toner images of black, yellow, cyan, magenta and the like are formed. The problem here, therefore, is that the printing speed for performing the color printing is approximately ¼ of the printing speed for performing black-and-white printing. In other words, color printing takes approximately four times longer than black-and-white printing. For this reason, the image forming apparatus with a color printing function is required to reduce the print time or, in other words, to increase the printing speed. Examples of image forming apparatuses that satisfy such requirements include a tandem-type color image forming apparatus.
Tandem-type color image forming apparatus has, for example, an intermediate transfer belt, to the surface of which a toner image formed on an image bearing member by an electrophotographic system is transferred primarily, and which secondarily transfers the primarily transferred toner image onto a transfer material such as a sheet. In other words, in the tandem-type color image forming apparatus, toner images of yellow (Y), magenta (M), cyan (C), and black (K) are primarily transferred successively onto the intermediate transfer belt to form toner images having a plurality of toner colors on the surface of the intermediate transfer belt, and the toner images having the plurality of toner colors are secondarily transferred onto the transfer material such as a sheet, whereby a color image is formed. In such an image forming apparatus, image forming units corresponding to the respective colors are arranged along the intermediate transfer belt in order to primarily transfer the toner images of these colors successively. The toner images of the four colors, YMCK, formed on the photosensitive drums of the respective image forming units are transferred successively (primary transfer), whereby color toner images composed of the toners, each having one of the plurality of colors are formed. The color toner images on the intermediate transfer belt, which are composed of the toners of the plurality of colors, are transferred onto the transfer material, such as a sheet, by a secondary transfer roller installed facing the intermediate transfer belt (secondary transfer). As a result, a color image is formed on the transfer material such as a sheet. In this manner, the toner images corresponding to the respective colors are formed on the image bearing members of the image forming units corresponding to the respective colors, and these toner images are successively transferred on the intermediate transfer belt. Subsequently, the color toner images composed of the toners of the plurality of colors are formed on the intermediate transfer belt and then transferred onto the transfer material such as a sheet, to form a color image. Therefore, the tandem-type color image forming apparatus realizes high-speed printing.
The image forming apparatus that uses the toners of a plurality of colors to form a color image, as described above, has a problem in which, although a color image is formed unlike the image forming apparatus that forms a single-colored image using a single-color toner, it is difficult to form an image with good quality. More specifically, the formed image tends to become, for example, uneven and dry. Moreover, a so-called defective image, which is an image with defects, is easily formed. This problem where a favorable image cannot be formed easily is particularly prominent when using an image forming method, e.g., the tandem-type color image forming apparatus, which forms an image by forming color toner images of the plurality of toners onto an intermediate transfer unit once and then transferring the formed toner images onto a recording medium.
Such a color image forming apparatus is an image forming apparatus for stacking visible images on an image transfer medium to form a color image, the image forming apparatus having an image forming unit that has, for example: a plurality of toner supply means for supplying different types of toners; at least one visible image forming means (developing means) for forming visible images with the toners on image forming media (photosensitive drums); and transfer means for transferring the visible images onto an image transfer medium (recording medium) while bringing the visible images into contact with the image transfer medium, wherein each of the toners accommodated in the plurality of toner supply means respectively includes an external additive, and when additive amounts Sn of the external additives contained in the toner within the respective toner supply means are represented by S1, S2, S3, S4, . . . . Sn sequentially in this order from an upstream side in a moving direction of a transfer material, S1>S2>S3>S4> . . . >Sn is established. It is expected that such an example can prevent the occurrence of a so-called reverse transfer phenomenon where, when a transfer step of stacking the toners on a sheet to form images is carried out before transferring the toner images, the toners adhere to the photosensitive drums.